In the manufacture of printed circuit boards, it is now commonplace to produce printed circuitry on both sides of a planar rigid or flexible insulating substrate. The manufacture of multi-layer printed circuits which consist of parallel, planar, alternating inner layers of insulating substrate material and conductive metal is of increased importance. The exposed outer sides of the laminated structure are provided with circuit patterns, as with double-sided boards, and the inner layers themselves may contain circuit patterns.
In double-sided and multi-layer printed circuit boards, it is necessary to provide conductive interconnection between and among the various layers and/or sides. This is commonly achieved by providing copper plated through-holes. Copper is provided in various ways such as by electroless or electrolytic deposition or combinations thereof. In terms of providing the desired circuit pattern on the board, the art has developed a variety of manufacturing sequences, many of which fall into the broad categories of subtractive or additive techniques. Common to the subtractive processes is the need to etch away (or subtract) metal to expose the desired circuit patterns. Additive processes, on the other hand, begin with clean dielectric substrate surfaces and build up thereon metallization in desired areas only, the desired areas being those not masked by a previously applied pattern of plating resist material. While avoiding the problems associated with the etching required in subtractive processes, additive processes have their own inherent difficulties in terms of the choice of resist materials, the ability to build up the full metallization thickness desired by electroless methods, the relatively long time periods required to electrolessly build the desired thickness' and weaknesses in the physical properties of most electroless copper deposits.
U.S. Pat. No. 4,897,118 (Ferrier et. al), whose teachings are incorporated herein by reference, reveals a process for selective metallization of a substrate in a predetermined desired pattern (i.e. additive technology). Ferrier et al. discusses additive technology, proposes certain improvements thereto, and gives a fair picture of the current state-of-the-art in this area. The current invention proposes improvements thereto which provide significant advantages in reducing the number of steps and chemicals involved in the fabrication thereby making the fabrication process more economical and feasible.
The prior art additive processes suffered from a variety of problems. Firstly, most plating masks currently used in the industry are strippable in alkaline solutions. Electroless copper baths are invariably alkaline, usually very alkaline, with pH's in excess of 12. Therefore, known plating resists have great difficulty in maintaining their integrity and adhesion to the board surface when subjected to plating in electroless copper baths, particularly when the long plating periods required by these techniques (8 to 24 hours) are taken into consideration. When the plating mask loses its integrity and/or adhesion to the surface, circuit definition fails. As one possible solution to this problem see U.S. Pat. No. 4,876,177 (Akahoshi et al.), the teachings of which are incorporated herein by reference, where the organic resist undergoes its final curing after chemical copper plating.
Many alternate techniques have been developed to additively and semi-additively produce circuit boards. As one such technique, Kukanskis et al. (U.S. Pat. No. 4,931,148), the teachings of which are incorporated herein by reference, reveals a process whereby an organic resist is used to pattern the surface of the printed circuit. The entire surface is subsequently activated then the resist surfaces are deactivated by the application of an alkaline solution. Plating then occurs chemically in the desired pattern. A second alternative is proposed in PCT Patent Application No. 9326145 (Knopp), the teachings of which are incorporated herein by reference. Knopp reveals a process for the production of printed circuit boards whereby circuits are etched, then a removable "desense" mask is applied and holes are drilled. The printed circuit board is then activated, the "desense" mask removed, and then the holes and circuit features are plated. For additional alternative methods see British Patent No. 1,259,304 (Photocircuits Corporation) and British Patent No. 1,207,631 (Technograph Limited), the teachings of which are incorporated herein by reference.
The current application proposes a new method for semi-additively producing printed circuit boards. The process proposed improves upon prior methods in several ways including reduction of processing steps and ease of use.